Synergistic effect of Al2O3-decorated reduced graphene oxide on microstructure and mechanical properties of 6061 aluminium alloy

In this study, Al6061 alloy matrix composites reinforced Al2O3-decorated reduced graphene oxide (Al2O3/RGO) with 0.1, 0.3 and 0.5 weight present (wt%) were successfully fabricated using high energy ball milling and hot extrusion techniques. The microstructures of these Al2O3/RGO/Al6061 aluminum matrix composites (Al MMCs) were characterized. The results showed that Al2O3/RGO were uniformly distributed within the Al6061 matrix and tightly bonded to the matrix. Al2O3 encapsulation on RGO surface would prevent the formation of Al4C3 brittle phase in matrix, ensuring that there was no reaction between the reinforcement and the matrix Al6061. Tensile strength and Vickers hardness tests demonstrated that the mechanical properties of Al MMCs significantly increased with addition of Al2O3/RGOs. Remarkably, Al MMCs with 0.1 wt% reinforcement showed tensile yield and tensile strengths of 270 MPa and 286 MPa, respectively, which were 49% and 43% higher than those of pure Al6061 prepared using the same process. Furthermore, the 0.1 wt% Al2O3/RGO composite also showed the best plastic deformation capability in considering of the strength.


List of symbols
The mass of the composite sample in air m 1 The mass of the same composite sample in distilled water ρ m The density of the composite ρ w The density of distilled water σ c The yield strength of Aluminum Metal Matrix Composite σ m The yield strength of Al6061 alloy σ LT The effect of load transfer strengthening σ GR The effect of grain refinement strengthening σ CTE The effect of coefficient of thermal expansion strengthening σ OR The effect of Orowan strengthening S The interfacial area between RGOs and Al6061 matrix A The cross-sectional area along the tensile direction The Hall Page coefficient d c The average grain size of Al 2 O 3 /RGO/Al6061 composite d m The average grain size of pure Al6061 σ Al 2 O 3

CTE
The effect of coefficient of thermal expansion strengthening between Al 2 O 3 and matrix σ RGO

CTE
The effect of coefficient of thermal expansion strengthening between Al 2 O 3 and RGO a The geometric constant G Al The shear modulus of Al B A constant between 4 (RGO) and 12 (Al 2 O 3 particles) b The burgers vector �α The difference in thermal expansion coefficients of the matrix and reinforcements T The difference between the test temperature (298 K) and the hot extrusion temperature (798 K) The diameter of the Al 2 O 3 D RGO The diameter of RGO Metal matrix composites have good physical and mechanical qualities, making them ideal for manufacturing lightweight structural components with high specific strength and specific modulus, which are widely employed in aerospace, aviation, automotive industries, and other disciplines 1,2 .Aluminum Metal Matrix Composite (Al MMC) are one of the most extensively utilized within this class of materials.They exhibit good ductility, durability, high specific strength and modulus, low coefficient of thermal expansion, excellent hightemperature properties, as well as good fatigue resistance and wear resistance 3 .Combined with their ease of processing, engineering reliability, and affordability, Al MMCs present favorable conditions for their utilization in engineering applications 4 .The reinforcement of Al MMCs primarily involves the use of oxide ceramic particles [5][6][7][8][9][10][11][12] , carbides [13][14][15][16][17][18][19][20] , nitrides 21 , and borides [22][23][24] , employing both non-in-situ and in-situ methods.
In recent year, graphene, renowned for its remarkable properties such as high strength, excellent thermal conductivity, and an extremely low coefficient of thermal expansion 25,26 , is considered an exceptional reinforcement in composite materials.Researchers have developed various methodologies to demonstrate that the incorporation of graphene enhances the mechanical, thermal, and tribological properties of metallic materials 27,28 .However, in the production of graphene/aluminum composites, graphene is prone to the easy breakage of the six-membered ring structure, poor wettability at the aluminum interface, and agglomeration 29 .Additionally, graphene readily reacts with the aluminum element in the aluminum alloy, leading to the formation of the brittle Al 4 C 3 phase, which results in a decline in the performance of the Al MMCs 30,31 .
Therefore, in this study, Al 2 O 3 /reduced graphene oxide (RGO) nanoparticles (Al 2 O 3 /RGO) with a specific layered structure were synthesized by loading Al 2 O 3 particles onto RGO through a hydrothermal process.The presence of Al 2 O 3 could inhibit the reaction between RGO and Al element to form Al 4 C 3 brittle phase.Subsequently, the Al MMCs were manufactured using a powder metallurgy method, incorporating them into the matrix Al6061 as the reinforcing phase.The aim is to investigate the impact of the Al 2 O 3 /RGO's synergistic reinforcement on the mechanical properties of the matrix Al6061.

Material
Al6061 alloy has outstanding mechanical characteristics because it is a precipitation solidified aluminum alloy composed primarily of magnesium and silicon.Therefore, in this investigation, Al6061 alloy powder was used as the foundation material.The content of other metallic elements in Al6061 is detailed in Table 1.The particle size distribution of Al6061 powder was analyzed using the Mastersizer 3000 laser diffraction particle size analyzer, as illustrated in Fig. 1a.The average diameter of Al6061 particles was determined to be 6.8 µm, and the morphology is shown in Fig. 1b.

Synthesis of RGO and Al 2 O 3 /RGO composites
In this study, RGO flakes were firstly synthesized from flake graphite using a modified hummers method 32 .To achieve a well-dispersed graphene oxide (GO) solution, 0.54 g of GO was initially dispersed in 30 ml of deionized water, stirred with a magnetic stirrer for 2 h, and subsequently sonicated for 30 min.Then, the preparation of process Al 2 O 3 /RGO involved mixing 5mmol glucose with 70ml of deionized water for 10 min, followed by the addition of 10mmol AlCl 3 •6H 2 O 3 and 10mmol NaAlO 2 , and stirring for 30 min.Subsequently, the welldispersed GO solution was added and the mixture was stirred for an additional 1 h.The stirred solution was then transferred to a 100ml stainless steel autoclave lined with Teflon and maintained at 150 °C for 24 h.Following The Al6061 powder was introduced to a high-speed mixer and blended with varying ratios (0.1, 0.3, and 0.5 wt%) of Al 2 O 3 /RGO for one hour to achieve a preliminary mixture.Following the blending process, the powder was transferred to a ball mill tank and subjected to planetary ball milling at 200 rpm for 10 h, alternating between 20 min of forward rotation and 20 min of reverse rotation.The milling was conducted with a ball to material ratio of 5:1, consisting of 20% of 10 mm balls, 60% of 8 mm balls, and 20% of 5 mm balls.
To create the preforms, the resulting mixture from ball milling was loaded into a 40 mm diameter extrusion barrel die and compressed at 300 MPa for 5 min.Once the extrusion die with a 16:1 extrusion ratio was mounted onto the extrusion barrel and securely fastened, the entire assembly, including the preforms, was transferred to a heating furnace.The furnace was preheated to 500 °C at a rate of 200 °C/h.After holding at 500 °C for 50 min, the process of hot extrusion commenced, yielding a composite aluminum alloy bar with a diameter of 10 mm.
For comparison, pure Al6061 samples without the incorporation of Al 2 O 3 /RGO were prepared using the same methodology.Figure 3 provides a schematic representation of the preparation process.

Characterization method
Density measurements were carried out on Al6061 and Al MMCs specimens using Archimedes' principle, and the density of the composite was calculated using the Eq. ( 1).
where m is the mass of the composite sample in air, m 1 is the mass of the same composite sample in distilled water and ρ w is the density of distilled water.The density of distilled water at 20 °C is 998 kg/m 3 .The microstructure of the obtained Al MMCs was observed using scanning electron microscopy (SEM) with the GeminiSEM 500.The phase composition was determined through X-ray diffraction (XRD) analysis using an energy dispersive X-ray spectrometer (EDS) and a Mini Flex 300/600.The specimens were exposed to Cu Kα radiation (0.15418 nm) with a scanning speed of 2°/min, and the 2θ scans were conducted in the range of 20° to 90°.Sheet samples of the alloy cross sections were manually crushed into thin foils less than 100um thick and punched into 3mm diameter plates with a puncher.The plates were electrolytically flattened with a twin-jet electropolishing machine before being inspected using a JEM-2100F transmission electron microscope (TEM) at 200 kV.The electrolyte was a solution of 10% perchloric acid in ethanol.
The hardness of extruded bars was evaluated using a Wilson|VH1102-01-0087 hardness tester in accordance with ISO 6507.The mechanical properties of the Al MMCs were assessed following ASTM E8M and ISO 6892-1 standards.Tensile specimens with a diameter of Φ5 were machined from the extruded bars and tested at room temperature using an E45.305 electronic universal testing machine (300 kN) at a constant rate of 0.1 mm/min.To ensure the statistical significance of the results, at least three samples were tested for each condition.After the tensile tests, the fracture surfaces and regions near the fractures of the specimens were examined using SEM.

Results and discussion
The morphology of GO is illustrated in Fig. 4a.GO exhibits a two-dimensional layered structure with undulating and wrinkled features, which demonstrate the high specific surface area of GO. Figure 4b shows the morphology of Al 2 O 3 /RGO, which many small particles adhered to layered RGO.Based on the EDS results (Fig. 4c at red circle area in Fig. 4b), additionally, it can be confirmed that the particles on the surface of RGO were Al 2 O 3 , indicating that Al 2 O 3 has been successfully loaded onto RGO.The Al 2 O 3 /RGO composite has an average diameter of ranging from 400 nm to 4 um.
As shown in Fig. 5a, it can be found that the addition of reinforcements has caused modifications in the morphology of the matrix composite.The initial spherical Al6061 powder has evolved into a polygonal shape with fractures spread throughout.This might be related to the flattening and cold welding of spherical particles during the ball milling procedure.As depicted in Fig. 5b, the EDS point scan findings of Zone A and B in Fig. 5a (indicated within a red box) show that the black strip are RGO, which are securely lodged in the aluminum alloy matrix by ball milling.
Figure 6 illustrates the changes in density and porosity of Al6061 and Al MMCs.It is evident that as the Al 2 O 3 / RGO ratio increase, the density of Al MMCs decreases while the porosity increases, aligning with the theoretical calculations.The increase in porosity, which can be attributed to factors such as layered structure of RGO and a considerable difference in the linear expansion coefficients between graphene and the Al6061 matrix 23 .
Figure 7 displays the transverse SEM microstructure of Al MMCs with varying mass percentages of Al 2 O 3 /RGO.The microstructure of the four specimens, produced using the same process but with different mass fractions of Al 2 O 3 /RGO, exhibited general similarities, as depicted in Fig. 7a-d.Notably, no discernible metallurgical defects such as shrinkage, bubbles, or cracks were observed.Following the addition of Al 2 O 3 /RGO reinforcement, fine black strips emerged within the Al6061 matrix, and the morphology is consistent with Fig. 5b., there was no brittle phase of Al 4 C 3 at the end of the hot extrusion preparation.This is attributed to the RGO, which is coated in Al 2 O 3 particles, exhibiting limited interaction with the Al6061 matrix, thereby hindering the formation of the Al 4 C 3 brittle phase.The texture coefficient of the alloy was calculated using the equation (Eq.2), where I is the intensity of diffraction peak, hkl denotes the (111), (200), or (222) orientation 33 .Figure 11 demonstrate the mechanical properties of Al MMCs with varying Al 2 O 3 /RGO contents.Figure 11a showcases the true stress-strain curves of the Al MMCs.As depicted in Fig. 11b and c, the addition of Al 2 O 3 /RGO led to a slight decrease in plasticity but an increase in strength for the Al MMCs.The incorporation of different amounts of reinforcement significantly enhanced the yield strength and tensile strength of the composites compared to pure Al6061.The maximum improvement was observed at a content of 0.1 wt%.Specifically, the yield strength increased by 49%, rising from 181 to 270 MPa, while the tensile strength improved by 43%, increasing from 200 to 286 MPa.Additionally, the reinforcement had a considerable impact on the elastic modulus of the composites, initially increasing and then declining.These results unequivocally demonstrate the substantial enhancement of the mechanical characteristics of the composites through the inclusion of the Al 2 O 3 / RGO phase.Similar findings were also reported by Saravanan 34 .
Figure 12 illustrates the fracture surfaces of Al6061 with the Al 2 O 3 /RGO phase.Dimples and tearing ridges were observed in the alloys, regardless of the presence of the Al 2 O 3 /RGO phase (Fig. 12a).Initially, the size of the dimples decreased with increasing content of the enhancement phase, but then increased, aligning with the variation in tensile elongation of the Al MMCs investigated.In Fig. 12b-d, the Al 2 O 3 /RGO phase can be seen being pulled out from the matrix, forming strip-like voids.This observation clearly demonstrates the effective strengthening effect of the enhancement phase.
The strengthening mechanisms of second-phase reinforced metal matrix composites can be attributed to many sources, such as load transfer strengthening, grain refinement strengthening, coefficient of thermal expansion (CTE) strengthening, and Orowan strengthening.In the present investigation, the reinforcements are twodimensional Al 2 O 3 -decorated RGO particles, thus necessitating the consideration of all four reinforcement mechanisms.Therefore, the multiple strengthening mechanisms operating in RGO and Al 2 O 3 synergistic reinforced Al MMC can be expressed as Eq. ( 3): where σ m is the yield strength of Al6061 (180MPa, in this study), and σ LT , σ GR ,σ CTE and σ OR is the effect of load transfer strengthening, grain refinement strengthening, CTE strengthening, and Orowan strengthening, respectively.
In this study, the load transfer strengthening of RGO provide the important part of the total strengthening.The effectiveness of the load transfer can be quantified using the load transfer model specific to RGO as outlined in Eq. ( 4) 35 : (3) where S is the interfacial area between RGOs and Al6061 matrix, A refers to the cross-sectional area along the tensile direction.According to Fig. 9, the average size of RGO is 20 µm × 20 µm × 10 nm , and the V RGO is volume percentage of RGO, calculated from weight content of RGO.The load transfer strength for 0.1, 0.3 and 0.5 wt% Al 2 O 3 /RGOs are 27, 76 and 134 MPa, respectively.High interfacial transfer efficiency is contingent upon the presence of elevated shear stress at the interface 36 .The presence of Al 2 O 3 on the surface of RGOs confines the RGO within the Al6061 matrix, tightly, thus increasing the critical shear stress value at the interface between the RGOs and the matrix.
Ball milling facilitates the incorporation of the Al 2 O 3 /RGO into the aluminum alloy powder, thereby enabling the suppression of grain growth during subsequent sintering and hot extrusion processes.According to the XRD results, the grain size of aluminum alloy composite materials can be calculated by the Williamson-Hall formula 14,37 .The yield strength of Al MMCs enhanced through grain refinement strengthening can be calculated by Hall-Petch relationship, as shown in Eq. ( 5).
where k is a constant (0.08 MPa m for Al alloy 38 ), d c and d m are the average grain size of Al 2 O 3 /RGO/Al6061 composite and pure Al6061, respectively.The strength improvements of grain refinement with 0.1, 0.3 and 0.5 wt% Al 2 O 3 /RGO are 48, 20 and 8MPa.
The residual stress induced by the mismatch in thermal expansion coefficients between the matrix and the particles may lead to the formation of dislocations around the particles, resulting in an increase in tensile strength.The thermal mismatch strengthening of the matrix can be quantified utilizing the subsequent Eq. ( 6) and ( 7) 39 .where a is the geometric constant (0.83) 40 , G Al is the shear modulus of Al (2.6 × 10 4 MPa), B is a constant between 4 (RGO) and 12(Al 2 O 3 particles) 41 , b is burgers vector (2.86 × 10 −10 m) 42 of Al, Δα is the difference in thermal expansion coefficients of the matrix and reinforcements (24 × 10 −6 K −1 , − 6 × 10 −6 K −1 and 7 × 10  8) 36,44 .The Orowan strengthening increment of Al 2 O 3 with different volume fraction is also given in Table 2.
The contributions of Al 2 O 3 /RGO reinforcement corresponding to the four strengthening mechanisms are shown in Table 2.The reinforcement effect of the composite material depends on the characteristics of the reinforcement.RGOs mainly play the role of load transfer, while the Al 2 O 3 phases play the role of Orowan strengthening.The synergistic effect of Al 2 O 3 and RGO improves interface bonding performance and enhances particle dislocation precipitation interaction, thereby enhance total composite material strength.It can be found the theoretically enhancement effect increases with the rise in the reinforcing phase content.However, a comparison with experimental findings reveals that when the reinforcing phase content surpasses 0.1 wt%, the calculated value become higher than the experimental value.The increase in particle volume fraction primarily leads the agglomeration of particles, reduction in the surface area of graphene interfacing with the matrix, and a decrease in interfacial bonding strength 45 .These factors collectively promote the initiation and propagation of cracks during tensile processes, consequently leading to a decline in the tensile strength of Al MMC with higher content of reinforcing phase.Thus, employing the suitable process techniques to control the size of the reinforcing phase and prevent their agglomeration 8,18 is important to the synergistic reinforcement effect of Al 2 O 3 and RGO on Al MMC.
Figure 13 presents an analysis of the fracture mechanism of Al MMCs to reveal their behavior under tensile forces.After the process of hot extrusion, the distribution of Al 2 O 3 /RGO in Al MMC demonstrates a certain degree of directionality.Initially, the matrix undergoes plastic deformation, resulting in the formation of numerous microcracks.As the tensile force increases, these microcracks progressively expand inward alongside the matrix, accompanied by an increase in dislocation movement.However, when these dislocations encounter the obstacles presented by Al 2 O 3 /RGO particles, their extension is impeded.The strengthening occurs when the movement of dislocations is hindered.Additionally, Al 2 O 3 particles were deposited onto RGO to increase its surface roughness and facilitate better interaction with the Al matrix.This could lead to the formation of an interlocking effect between the Al 2 O 3 /RGO reinforcement and the Al6061 matrix, which further strengthen the material 46 .Furthermore, due to the disparate thermal expansion coefficients of Al6061 and Al 2 O 3 /RGO nanoparticles, strain fields are created around the Al 2 O 3 /RGO during the cooling process following hot extrusion.www.nature.com/scientificreports/These strain fields act as barriers to dislocation movement during stretching.Consequently, a higher load is required to transfer these dislocations around the strain field.The fracture morphology of the stretched specimen in Fig. 13 further supports and elucidates this phenomenon.

Figure 1 .
Figure 1.Particle diameter distribution (a) and the morphology (b) of Al6061 powder.

( 2 )Figure 5 .
Figure 5.The typical SEM image of Al6061 powder with 0.1 wt.% Al 2 O 3 /RGO after ball mill (a) and EDS point scan results of (a) at A and B area (b).

Figure 10
Figure 10 depicts the average microhardness of Al MMCs.As the Al 2 O 3 /RGO content in the Al 6061 matrix rises, the microhardness values of the Al MMCs experience a substantial boost.Specifically, their hardness escalates from 69.02 to 97.78 Hv, marking a remarkable 41.6% increase.This clearly demonstrates the beneficial impact of Al 2 O 3 /RGO on Al6061.Figure11demonstrate the mechanical properties of Al MMCs with varying Al 2 O 3 /RGO contents.Figure11ashowcases the true stress-strain curves of the Al MMCs.As depicted in Fig.11b and c, the addition of Al 2 O 3 /RGO led to a slight decrease in plasticity but an increase in strength for the Al MMCs.The incorporation of different amounts of reinforcement significantly enhanced the yield strength and tensile strength of the composites compared to pure Al6061.The maximum improvement was observed at a content of 0.1 wt%.Specifically, the yield strength increased by 49%, rising from 181 to 270 MPa, while the tensile strength improved by 43%, increasing from 200 to 286 MPa.Additionally, the reinforcement had a considerable impact on the elastic modulus of the composites, initially increasing and then declining.These results unequivocally demonstrate the substantial enhancement of the mechanical characteristics of the composites through the inclusion of the Al 2 O 3 / RGO phase.Similar findings were also reported by Saravanan34 .Figure12illustrates the fracture surfaces of Al6061 with the Al 2 O 3 /RGO phase.Dimples and tearing ridges were observed in the alloys, regardless of the presence of the Al 2 O 3 /RGO phase (Fig.12a).Initially, the size of the dimples decreased with increasing content of the enhancement phase, but then increased, aligning with the variation in tensile elongation of the Al MMCs investigated.In Fig.12b-d, the Al 2 O 3 /RGO phase can be seen being pulled out from the matrix, forming strip-like voids.This observation clearly demonstrates the effective strengthening effect of the enhancement phase.The strengthening mechanisms of second-phase reinforced metal matrix composites can be attributed to many sources, such as load transfer strengthening, grain refinement strengthening, coefficient of thermal expansion (CTE) strengthening, and Orowan strengthening.In the present investigation, the reinforcements are twodimensional Al 2 O 3 -decorated RGO particles, thus necessitating the consideration of all four reinforcement mechanisms.Therefore, the multiple strengthening mechanisms operating in RGO and Al 2 O 3 synergistic reinforced Al MMC can be expressed as Eq.(3):

Figure 9 .
Figure 9. Bright-field TEM and EDS images of 0.1 wt% Al 2 O 3 /RGO/Al6061 composites: the high magnification image showing Al 2 O 3 /RGO fragments embedded in the composites (a), low-magnification images showing Al 2 O 3 /RGO in composites (b), the EDS mapping images of 0.1 wt% Al 2 O 3 /RGO/Al6061 composites in the redframed area (c) of (b).

Table 2 .Figure 13 .
Figure 13.Schematic diagram of the deformation mechanism of the Al MMCs.

Table 1 .
Chemical composition of Al6061 alloy (wt%).reaction, the sample was washed with deionized water and ethanol, and then dried at 60 °C for 24 h.The resulting Al 2 O 3 /RGO complex was crushed to produce Al 2 O 3 /RGO.Synthesis of Al 2 O 3 /RGO is shown in Fig.2. the

Prepare of RGO, Al 2 O 3 /RGO and Al 2 O 3 /RGO/Al6061 composite
43 K −1 for aluminum, RGO and Al 2 O 3 )43. Δ the difference between the test temperature (298 K) and the hot extrusion temperature (798 K), D Al 2 O 3 and D RGO is the diameter of the Al 2 O 3 and RGO, V Al 2 O 3 is volume percentage of Al 2 O 3 .The CTE strengthening increment of RGO and Al 2 O 3 with different volume fraction is given in Table2.Because of the small dimensions of Al 2 O 3 particles adhered to RGO, the Orowan strengthening effect of Al 2 O 3 particles can be anticipated by applying the Orowan strengthening model, as shown in Eq. (